The present invention relates to a gas generator for an airbag, particularly in a motor vehicle, consisting of a pressure vessel for receiving a highly compressed gas under high pressure, with at least one opening in the pressure vessel which is closed by a seal and with a release mechanism.
In the current state of the art, essentially two different types of gas generator for airbags are known. The first type is of a purely pyrotechnical nature, that is to say a gas is available in chemically bonded form, preferably in the form of granules or powder pressed into tablets and is explosively released by ignition of a propellant. As propellants, substances are used which are based on sodium nitride (NAN.sub.3) and an oxidation medium (for example, iron oxide), which, when combustion occurs, release the chemically bonded nitrogen, which is thereby available as gas and thereafter takes up a significantly greater volume. The chemically stored gas or oxidators used are therefore specifically mixed and processed together in such proportions and such compositions that the release of the gas finally takes place as a controlled combustion, that is to say certainly very rapidly, as an airbag must fill with gas within a fraction of a second, but on the other hand, this cannot be in the form of uncontrolled combustion, which could seriously damage the airbag and the other surroundings.
These pyrotechnical gas generators have the disadvantage that when ignited, significant quantities of solid fuel remnants are produced, which must be contained during the violent combustion process to avoid them coming into contact with the relatively thin walled and delicate material of the airbag, which they could otherwise perforate and thereby risk burning the occupants of the vehicle. This necessitates integration of suitable filters which results in proportionately high costs for the gas generator. Gas generators with alternative chemical substances and organically based compositions have the disadvantage that they are not always very stable and their properties change in time because of the effects of heat. As an airbag built into a motor vehicle may perhaps only be used after fifteen years, its functional capability can be significantly impaired by this. Furthermore, organic propellants produce large amounts of toxic gases, particularly carbon monoxide (CO).
Due to the aforementioned reasons, gas generators were developed in the past composed essentially of a pressure vessel which contains highly compressed gas and are provided with an opening with a seal. Furthermore, for such gas generators a release mechanism is provided, which, for example, penetrates or otherwise destroys a membrane which generally seals the exit opening of such a pressure vessel, so that the gas which is under high pressure can exit and fill the airbag. In conventional airbags for drivers or passengers in a motor vehicle, the volume of this type of high pressure vessel is usually between 200 and 400 cm.sup.3, and the gas is compressed to a pressure typically between 200 and 300 bar in this pressure vessel, so that under normal pressure, or the slight high pressure in the finally filled airbag, this gas fills a volume of between about 50 and 150 liters. Naturally, deviations from these typical values are possible, according to usage.
In a gas generator which exclusively contains compressed gas in a pressure vessel, which is released when an accident occurs, the disadvantage arises that the expansion of the gas, which occurs in a fraction of a second and is thus practically adiabatic, drastically lowers the temperature thereof, so that at first, at normal pressure, it only occupies a comparatively small volume which would not be sufficient to satisfactorily fill the airbag if the pressure vessel were not designed to be correspondingly larger. The latter would involve problems with space. Furthermore, with such gas generators, there is the significant disadvantage that the gas development occurs degressively, whereas for optimal and timely filling of an airbag, a progressive gas development in the first phases is necessary instead.
For this reason, apart from the purely pyrotechnical gas generators, in practice so-called hybrid generators have proved themselves, which, besides a store for highly compressed gas also have a pyrotechnical charge which is however mainly for heating purposes and less for gas development, so that the adiabatically expanding gas is heated at the same time as it expands and thus sufficiently fills the volume of the airbag. Compared to a purely pyrotechnical gas generator, such a hybrid generator has the advantage that the pyrotechnical charge has to be exclusively for heating purposes and in that there is a larger choice of suitable pyrotechnical heating media. The amount of pyrotechnical charge can be kept significantly lower than in the case of a purely pyrotechnical gas generator, so that only a few solid fuel remnants are produced.
By selective layout and combination of the pyrotechnical heating charge with respect to the quantity as well as with respect to the combustion characteristic, an optimum filling procedure for the airbag can be achieved for the respective motor vehicle requirement.
Obviously, the use of such an airbag is not restricted to steering wheel bosses, nor indeed to motor vehicles, but could for example be installed in boats or ships or aircraft, and generally anywhere that a large volume of gas is rapidly required for a short time.
The state of the art closest to the present invention is a gas generator with a pressure vessel for highly compressed gas, in which the previously described hybrid solution is preferred but not absolutely necessary.
With known gas generators of this type, as already described, the gas is placed in a pressure vessel at a pressure of between 200 and 300 bar, typically at about 250 bar. This pressure vessel is then closed and an exit opening for emergencies is sealed by a membrane, wherein a release mechanism destroys the sealing membrane in case of an accident or other airbag release requirement and thus liberates the opening for the exit of the gas. As previously mentioned, a heating charge can also been ignited at the same time, which simultaneously heats the exiting gas. Clearly, a membrane of this type has to be very carefully designed, manufactured and installed in the pressure vessel. This is because on the one hand, this membrane has to tightly seal the pressure vessel against a pressure of approximately 250 bar for many years, and on the other hand it also has to give way to the release mechanism if an accident or other airbag release requirement occurs. The development and manufacture of such membranes is thus complex and expensive. The release mechanism also has to be designed in a correspondingly complex manner, so that it destroys the membrane with a preset minimum force and if possible also ignites the heating charge.